The present invention relates to a laser delivering two waves at different frequencies, which are close to each other. It is suitable for use in all fields of measurement using a heterodyne effect, particularly in metrology; it is also applicable in optical spectrometry.
Lasers are already known delivering two waves at different frequencies, having mutually orthogonal linear polarizations, comprising a lasing medium placed in a laser resonating cavity defined by mirrors and associated with means for exciting the laser medium and comprising birefringent means placed on the path of the two waves.
The article "Two frequency gas lasers in mutually orthogonal transverse magnetic fields" by Gudelev et al, Sov. J. Quantum electron 18(2), February 1988, page 166 discloses a helium-neon gas discharge laser of the above type, in which separate zones of the active lasing medium are subjected to mutually orthogonal magnetic fields, the birefringent means being formed by one of two windows defining the cavity, subjected to a compression stress.
With such a laser, in effect, two waves can be obtained having a frequency difference related to the phase linear anisotropy, which may reach 50 MHz in the example given in the above document, using a resonant cavity 31 cm long.
Apart from its complexity, the laser described in the above document, as well as other lasers already proposed for generating two waves at different frequencies, have drawbacks. Since the optical path is the same for the two waves, the oscillation modes are very highly coupled and it is difficult to obtain simultaneous oscillations in the two inherent states without mutual interaction. Adjustment of the frequency difference is difficult and unreliable, for it is achieved by modifying a stress which is difficult to evaluate rather than by moving of a component.